1. Field of the Invention
The present invention generally relates to ophthalmic lens polishing laps, and more specifically to a castable lens polishing lap that can be deformed and cast to conform to different prescription lens curvatures.
2. Description of the Related Art
Lenses for certain types of eyeglasses are manufactured by utilizing a lens blank which is cast with a completed front curvature and an unfinished back surface. The lens' front surface is "blocked" to a metal mandrel by a variety of techniques such as a layer of plastic. The blocked lens is placed in a lathe or generator to machine the back surface of the lens. As shown in FIG. 1, existing generators such as the SG-8 produced by Gerber Scientific Products, Inc., machine the back surface of a lens 2 to roughly a prescription curvature 4. The rough cut 6 is characterized by errors in curvature, commonly called form errors, and roughness errors, approximately 30 .mu.m peak-to-valley. The rough cut 6 may or may not accurately represent the desired prescription 4, and thus is not considered to form the prescription but merely to provide a rough approximation. The surface that is produced is either spherical or toric (rotationally non-symmetric) in shape and requires a lapping and polishing operation to first form the prescribed curvature and then to smooth the surface.
Industry practice is to use hard laps, typically aluminum, with abrasive and soft pads to respectively fine and polish the back surface of the lens. The laps are pre-machined with the precise major and minor axis curvatures specified by a particular prescription. The lap and lens are placed in a cylinder machine such as the Opti-Speed.RTM. 2100 Surface and Polish Machine with their respective major and minor axes precisely aligned to each other. The cylinder machine rubs the pad against the back surface of the lens in a controlled manner to both grind the lap's curvature into the lens to remove form errors and to smooth the back surface. First, a highly abrasive pad is attached to the lap and rubbed against the lens for 1 minute. The high abrasive pad is replaced with a less abrasive pad which rubbed against the lens for 2 minutes. Finally, a felt polishing pad is saturated with an aluminum oxide liquid abrasive and used to polish the lens for 4 minutes.
A lens laboratory will typically have thousands of metal laps to produce the spherical shapes of varying radii and the many combinations of toric shapes that are required. Furthermore, the laps are only available with a resolution of 0.125 Diopters between successive laps. Thus, the actual prescription ground into the back surface of the lens may be up to 0.0625 Diopters different then the desired prescription. The purchase and maintenance of thousands of metal laps is expensive and requires a large amount of space. Furthermore, the aluminum laps become damaged over time which changes their effective curvature.
Some generators are able to produce both the lens and a plastic lap, which has a convex surface that is complementary to the concave back surface of the lens. A separate lap must still be generated for each radii and combination of toric shapes. The plastic laps are less expensive then the metal laps and do not require the same storage space. However, the production of the plastic lap takes time which prevents the generator from being utilized to machine lenses. Furthermore, the precision of the plastic lap is limited to the rough cut precision of the lens' back surface.
U.S. Pat. No. 5,345,725 "Variable Pitch Lapping Block for Polishing Lenses" to Anthony discloses an expandable rubber bladder whose curvature is adjusted by varying the air pressure on the inner surface of the bladder. The bladder is held against a lens' unfinished surface and pressurized until it conforms to the curvature of the lens. Stretching the bladder creates spring forces or aberrations which vary across its surface. As a result, rubbing the bladder against the lens creates waves in the surface of the lens. Furthermore, the bladder can change shape during the polishing action.